Why are man's days compared to grass?
Radak: His days are few and pass quickly, just like grass dries quickly. If his days were long, he could improve his ways if he sinned in his youth. Now, even if he grows old according to his nature 1 , and he begins to fix his ways, he will not finish, for he will die "like a blossom of the field", which sprouts and passes (withers) quickly.
Malbim: Grass dries faster than other vegetation. So man's days are short and he has no time to acquire perfection.
I.e. he continues in his bad ways until the start of old age. (PF)
What is the comparison to a blossom of the field?
Malbim: A blossom has no substance, even when it is its 'father' (the tree). So man's life is folly and nothingness.
